(°C)T | ||||
500 | 0–0.2 | 0–0.07 | ||
700 | 0–0.2 | 0–0.1 | ||
800 | 0–0.2 | 0–0.1 |
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共查询到20条相似文献,搜索用时 15 毫秒
1.
Zoning and recrystallization of phengitic micas: implications for metamorphic equilibration 总被引:2,自引:0,他引:2
T. J. Dempster 《Contributions to Mineralogy and Petrology》1992,109(4):526-537
White micas (phengites) in the metasediments of the Scottish Dalradian display a large range of compositions within single samples. The variations in the composition of these phengites are strongly controlled by their structural age, with early fabrics containing a paragonite-poor, celadonite-rich phengite whereas in later fabrics the micas are generally paragonite-rich and celadonite-poor. Retrograde phengite growth, identified using back scattered electron imaging, occurs as celadonite-rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica-plagioclase grain boundaries. The presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient. It is proposed that diffusion-controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible. This closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism. Many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation/recrystallization controlled equilibria. 相似文献
2.
3.
Evaluation of thermobarometers for garnet peridotites 总被引:1,自引:0,他引:1
The accuracy and precision of a large number of combinations of geothermometers and geo-barometers for garnet lherzolites have been evaluated with a suite of well-equilibrated xenoliths from kimberlites of northern Lesotho. Accuracy was tested by comparison of P-T estimates for a diamond-bearing and a graphite-bearing xenolith with the experimentally determined diamond-graphite univariant curve and by comparison of P-T estimates for phlogopite-bearing xenoliths to the high-temperature stability limit of phlogopite (Eggler and Wendlandt, 1979). Precision was evaluated by measuring the scatter of P-T estimates for each of four xenoliths from a wide range of P and T when many point analyses of the constituent minerals are used for P-T estimation. A thermobarometer composed of the uncorrected diopside-enstatite miscibility gap of Lindsley and Dixon (1976), combined with the uncorrected isopleths for aluminum in enstatite coexisting with pyrope of MacGregor (1974), is most satisfactory. Correction schemes such as those of Wells (1977) and Wood (1974) will ultimately provide a better means of P-T estimation, but at the present stage of development they serve to decrease precision without a demonstrable increase in accuracy. Thermometers based on exchange reactions are imprecise because of variable and unknown in minerals and xenoliths. The inflection observed in the northern Lesotho paleogeotherm cannot be an artifact of the method of temperature estimation. 相似文献
4.
5.
J.S. Delaney J.V. Smith D.A. Carswell J.B. Dawson 《Geochimica et cosmochimica acta》1980,44(6):857-872
Micas from coarse granular Iherzolites in S. African kimberlites may be separated into two groups; those showing primary textural relationships with coexisting silicates and those with secondary, alteration relationships with other silicates. Primary-textured micas form a tight cluster with a mean composition from 10 coarse garnet lherzolites of: SiO2 41.0, TiO2 0.18, Al2O3 13.5, Cr2O3 0.82, total Fe as FeO 2.60, MnO 0.02, MgO 26.0, NiO 0.22, CaO 0.01, BaO 0.29, Na2O 0.31, K2O 10.0, Rb2O 0.028, Cl 0.08, F 0.43 wt%. Primary-textured micas in aggregates with clinopyroxene have higher TiO2 and four specimens which look similar to the primary group but have textural ambiguities have still higher TiO2Micas with secondary textures have wide ranges of composition which may be correlated with details of the textural parageneses. Micas from kelyphitic rims around garnets tend to be Cr-rich while those from veinlets are Cr-poor. Both groups tend to have higher FeO and TiO2 than the primary group. Micas produced by alteration of, or filling veinlets through, orthopyroxene have a wide compositional range which overlaps that of the primary-textured micas, especially for harzburgite specimens.The primary-textured micas show a positive correlation with coexisting pyroxenes for MgO/(MgO + FeO) and TiO2, but not for Cr2O3. Secondary-textured micas do not show correlations with coexisting pyroxenes for any elements.The ‘primary-metasomatic’ micas described by Harte and Gurney (1975) and metasomatic and other micas described by Boettcher et al. (1979) and Boettcher and O'neil (1979) are richer in FeO and TiO2 than the present primary-textured micas, and are attributed to crystallization from fractionated fluids. 相似文献
6.
Mauro Gemmi Marco Merlini Alessandro Pavese Nadia Curetti 《Physics and Chemistry of Minerals》2008,35(7):367-379
Phengite samples (2M
1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray
diffraction. The measured volume thermal expansion of 2M
1 phengite (<α
V> ≈ 36.6 × 10−6 K−1) was systematically greater than <α
V> of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle
at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed
in 2M
1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating
of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and
by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate
phase derived from 2M
1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect
to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows
two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not
proceed to completion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
7.
Jadvyga Rimšaite 《Contributions to Mineralogy and Petrology》1964,10(2):152-183
Micas from magmatic and metamorphic rocks differ from one another in chemical composition and in trace element content. The
chemical composition of micas is discussed in relation to their occurrence, paragenesis and sequence of crystallization. On
the basis of previous studies of the relationship between the physical properties and the chemical composition of 34 chemically
analysed micas, reliable physical methods have been established which permit identification of different mica varieties in
the same rock. Structural formulae and trace element content of micas from basic and granitic rocks, as well as from skarns,
schists, ortho- and paragneisses are discussed. The relationship between the components of the tetrahedral and octahedral
layers and of the interlayer are illustrated as ratios.
Poorly differentiated, hybrid and metasomatic rocks often contain more than one variety of mica. Some prophyritic basalts
and lamprophyres contain an early phlogopite which is paragenetically related to pyroxene phenocrysts and late biotite which
occurs in the groundmass and in the fractures as a result of the crystallization of residual magma. The biotitemuscovite assemblage
was observed in granodiorites, quartz-monzonites, schists and gneisses. In the albite-K-feldspar granites, muscovite predominates
and the biotite is usually altered.
The chemical composition of micas from metamorphic rocks depends on the grade of metamorphism and on the nature of associated
minerals. The biotite from paragneisses contains considerable quantities of octahedral alumina. Pre-metamorphic micas show
variable deficiencies of the (OH, F) group. The micas are useful minerals in determining the degree of differentiation and
subsequent alteration of igneous rocks.
The present study was carried out on the basis of 34 recent complete chemical analyses andca 100 X-ray fluorescence analyses.
Dedicated to Professor Dr.Carl W. Correns on the occasion of his 70th birthday. 相似文献
8.
《International Geology Review》2012,54(11):1504-1507
There is a close analogy between dehydration and loss of argon curves obtained by laboratory heating of muscovite (figs. 2 and 3). Potassium atoms lie in potential cleavage planes of the muscovite. When K40 atoms disintegrate the resulting argon atoms are driven into the mica structure by recoil, except those whose paths make a small angle with the cleavage plane. These remain in the "unstable zone" and may escape from the muscovite. This introduces an element of uncertainty in K/A absolute age figures from mica. -- F. E. Ingerson. 相似文献
9.
Gold decoration and platinum-carbon-shadowing techniques of electron microscopy have been applied to study the surface microtopographs of noncleavage (001) faces of porphyroblastic white micas and sericite crystals collected from the chlorite, biotite, and garnet zones in the Shiragayama area in the Sanbagawa metamorphic terrain, Shikoku, Japan. Two different types of surface microtopograph have been observed; parallel step system and saw-tooth step system. The former is interpreted as representing the surface microtopographs formed either by growth or under near equilibrium conditions, the latter either by dissolution or under rigorous kinetic conditions. The former has been observed on porphyroblastic white micas occurring in the middle portion of each metamorphic zone and on all sericite crystals, the latter on most sericites and on porphyroblastic white micas occurring along the boundaries of the neighboring metamorphic zones. It is suggested that the observed variations in the surface microtopographic characteristics are due to Ostwald ripening during a kind of sintering process in which interstitial water plays an essential role, and that the drastic changes along the metamorphic zone boundaries are due to dehydration reactions. Retrogressive metamorphism is considered to have an almost negligible effect, if any, upon the surface microtopographs. 相似文献
10.
《International Geology Review》2012,54(10):1225-1226
Thermal karst, exposed in mine shafts, has been significantly developed in the commercial stratum of oil shales. Geochemical investigations in this karst have for the first time established the secondary changes in the kerogen of the oil shales, namely, an anomalous content of bituminoid A + C, the presence of processes of aromatization, the significant amount of hydrocarbon compounds and humic acids, and changes in the elemental composition of the residual organic matter (in particular, an increase in the C/H ratio). The secondary changes suggest that a favorable geochemical and thermobaric environment existed in the thermal karst for intense bitumen formation. The results obtained are of interest in the study of the kerogen of oil shales as a possible parental source for oil and gas. —Authors. 相似文献
11.
Milan Rieder Miroslav Huka Dagmar Kučerová Luděk Minařík Jiří Obermajer Pavel Povondra 《Contributions to Mineralogy and Petrology》1970,27(2):131-158
Compositions of natural lithium-iron micas are approximated best by the sidero-phyllite-polylithionite join. These micas contain little or no magnesium and manganese. Their octahedral sheets contain close to two trivalent cations (mainly aluminum) in small crystallographic sites and a variable quantity of lithium and R+2 (mainly iron) in large sites. Octahedral vacancies are situated mostly in large sites. Lithium and R+2 approach a 44 replacement relationship in micas with octahedral occupancy close to six. Lithium and fluorine show a good positive correlation (small excess of fluorine over lithium), which indicates a crystallochemical association between them. There is a less distinct positive correlation between lithium and R+4.Based on simplifications, a calculation shows that about two-thirds of octahedral vacancies are caused by substitutions within the octahedral sheet, one-third, by tetrahedral substitutions. Different methods of calculating the crystallochemical formula yield slightly different numbers of octahedral vacancies, but do not affect the mica's position in plots of physical parameters against composition. If a crystallochemical formula is calculated from analysis of a mica contaminated with quartz, topaz, or feldspar, the apparent number of octahedral vacancies increases; such a formula exhibits unusual behavior in composition plots. 相似文献
12.
Freshly cleaved (001) natural muscovite was chemically characterized by X-ray Photoelectron Spectroscopy (XPS). The investigated
muscovite crystals have composition ranging from pure end member to phengitic muscovite. XPS provides experimental evidence
that the chemical composition of the near-surface region differs from the bulk. This difference is not due to analytical problems,
but to extreme surface sensitivity of XPS. Depth profiling was also carried out after tilting of 60° or 70° of the sample
along an axis perpendicular to [001]. After tilting of phengitic muscovite, the concentration of Al increases and of K decreases,
and therefore Al is the topmost monolayer. Repulsion between the octahedral and tetrahedral sheets, stacking faults or more
probably interlayered phases (gibbsite-like) are responsible for the cleavage. Possible perturbations of surface chemistry
due to X-ray radiation and Ar+ ion sputtering have also been investigated. X-ray radiation is negligeable, but Ar+ ion sputtering introduces important structural modifications, perturbation of the chemical environment of elements and minor
chemical variations.
Received: 13 November 1996 / Revised, accepted: 21 March 1997 相似文献
13.
The stability of synthetic jadeite-acmite pyroxene coexisting with albite and quartz has been determined at 600, 700, and 900° C. The end-member reaction: albite = jadeite + quartz has been determined to lie between 1.67 and 1.70 GPa at 600° C, 1.88 and 1.90 GPa at 700° C, and 2.44 and 2.48 GPa at 900° C. Jd78Acm22 + quartz is stable above 1.58, 1.78, and 2.33 GPa at 600, 700, and 900° C, respectively. Jd61Acm39 + quartz is stable above 1.47, 1.67, and 2.18 GPa at 600, 700, and 900° C, respectively. Addition of as much as 40% of acmite component in jadeite extends pyroxene stability by less than 300 MPa at 900° C. Unit-cell parameters measured for synthetic jadeite-acmite pyroxenes indicate linear volume-composition relations. The data are consistent with ideal mixing in jadeite-acmite solutions. 相似文献
14.
A.E. Beswick 《Geochimica et cosmochimica acta》1973,37(2):183-208
K and Rb distributions between aqueous alkali chloride vapour phase (0.7 molar) and coexisting phlogopites and sanidines have been investigated in the range 500 to 800°C at 2000 kg/cm2 total pressure.Complete solid solution of RbMg3AlSi3O10(OH)2 in KMg3AlSi3O10(OH)2 exists at and above 700°C. At 500°C a possible miscibility gap between approximately 0.2 and 0.6 mole fraction of the Rb end-member is indicated.Only limited solid solution of Rb AlSi3O8 in KAlSi3O8 has been found at all temperatures investigated.Distribution coefficients, expressed as in solid/(Rb/K) in vapour, are appreciably temperature-dependent but at each temperature are independent of composition for low Rb end-member mole fractions in the solids. The determined values and their approximate Rb end-member mole fraction () ranges of constancy are summarized as follows: (°C)
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